Situation: A 70-year-old man with benign prostatic hyperplasia (BPH) has had a weak stream and dribbling for several months. He comes to the emergency room because he has not been able to void for 12 hours. His lower abdomen is tender, a firm, rounded mass is felt above the pubis, and his creatinine is 2.1 mg/dL (186 µmol/L). The physician orders insertion of an indwelling urinary catheter. He says that on the evening before he stopped voiding, he took several over-the-counter products for a cold and for poor sleep. Which product MOST likely precipitated his acute retention?

Diphenhydramine, the sedating antihistamine in most over-the-counter sleep aids and many cold remedies, has strong anticholinergic effects that weaken detrusor contraction and can tip an enlarged prostate into acute urinary retention; in older adults it also causes confusion and falls. Paracetamol, saline nasal spray, and guaifenesin do not act on the bladder or prostate.

Why diphenhydramine precipitated acute urinary retention

Diphenhydramine is a first-generation, sedating antihistamine. In addition to blocking histamine H1 receptors, it blocks muscarinic acetylcholine receptors in the bladder wall. The detrusor muscle depends on parasympathetic muscarinic stimulation to generate a strong, coordinated contraction during voiding. When diphenhydramine blocks those receptors, detrusor contractility falls. A man with BPH already has increased urethral resistance from the enlarged prostate, so he depends on a forceful detrusor contraction to overcome that resistance. Adding anticholinergic-induced detrusor weakness can therefore convert chronic partial obstruction into complete, painful retention.

Key point! Acute urinary retention in an older man with BPH is often drug-precipitated, and drugs with anticholinergic potential are among the most common triggers.

The creatinine of 2.1 mg/dL (186 µmol/L) suggests that the prolonged high-pressure retention has begun to impair renal clearance. In obstructive uropathy, sustained elevation of intravesical pressure is transmitted upward to the ureters and renal collecting system, reducing glomerular filtration. Relief of the obstruction by catheterization is the immediate priority, but identifying and stopping the offending drug is essential to prevent recurrence.

Why the other options are less likely

Product	Mechanism	Effect on voiding
Paracetamol	Weak cyclooxygenase inhibition; no muscarinic blockade	No direct effect on detrusor or prostate smooth muscle
Saline nasal spray	Topical isotonic sodium chloride; negligible systemic absorption	No bladder or prostate effect
Guaifenesin	Expectorant that increases respiratory tract secretions	No anticholinergic or adrenergic action on the lower urinary tract

Broader drug-safety context

The literature on adverse drug reactions involving the lower urinary tract identifies anticholinergics and drugs with anticholinergic potential as a major iatrogenic cause of urinary retention ^[1]. This is not limited to drugs prescribed for overactive bladder; many over-the-counter sleep aids, cold preparations, and older antihistamines share the same muscarinic-blocking property. In older adults, the risk is amplified because baseline detrusor reserve is reduced, prostatic obstruction is common, and polypharmacy increases the total anticholinergic burden.

Watch out! A patient may not recognize that a “sleep aid” and a “cold tablet” contain the same active ingredient, diphenhydramine. Taking several products together can produce a cumulative anticholinergic load even when each individual dose seems small.

Clinical application

When an older man presents with acute urinary retention, the medication history should specifically ask about over-the-counter sleep aids, antihistamines, and combination cold remedies. Diphenhydramine and similar sedating antihistamines should be considered probable precipitants until proven otherwise. After catheterization decompresses the bladder, the drug should be discontinued, and future avoidance should be documented. In older adults, diphenhydramine also carries central nervous system risks such as confusion and falls, so removal from the regimen benefits more than just the urinary tract.

The combination of BPH-related outlet obstruction and drug-induced detrusor weakness is a classic two-hit mechanism for acute retention. The prostate narrows the outlet, while the anticholinergic drug removes the force needed to push urine through it. Recognizing this interaction allows the nurse to anticipate which patients are at risk and to intervene before complete obstruction develops.